1. Field of the Invention
The present invention relates to an optical device, an optical tunable filter, an optical tunable filter module, and an optical spectrum analyzer.
2. Description of the Related Art
For example, an optical tunable filter for separating only light having a specific wavelength from light having a plurality of wavelengths has been known as an optical device. For instance, U.S. Pat. No. 6,747,775 (Patent Document 1) discloses such an optical tunable filter.
An optical tunable filter disclosed by Patent Document 1 has a movable portion in the form of a plate, support substrates, and reflection films formed on opposed surfaces of the movable portion and the support substrate, respectively. The movable portion is movable in a thickness direction thereof. One of the reflection films is formed on a surface of the movable portion near the support substrate, and the other of the reflection films is formed on a surface of the support substrate near the movable portion.
Further, drive electrodes are provided on the support substrate. When a potential difference is generated between the drive electrodes and the movable portion, an electrostatic attraction force is generated between the drive electrodes and the movable portion so as to move the movable portion. Thus, a clearance between two reflection films can be adjusted by moving the movable portion. When light having a plurality of wavelengths is introduced into the clearance, an interference effect is caused so as to emit only light having a wavelength corresponding to the clearance.
In order to effectively cause an interference effect, it is necessary to accurately set a distance between the movable portion and the support substrate or to enhance parallelism between the movable portion and the support substrate. Accordingly, in order to detect parallelism between the movable portion and the support substrate or a distance between the movable portion and the support substrate, it is necessary to provide a plurality of drive electrodes and a plurality of detection electrodes provided on the support substrate so as to correspond to the drive electrodes. Parallelism between the movable portion and the support substrate or a distance between the movable portion and the support substrate is detected based on a capacitance between the movable portion and the detection electrodes.
However, in the optical tunable filter of Patent Document 1, the support substrate has a flat surface near the movable portion. Accordingly, the detection electrodes are provided on the same plane as the drive electrodes. Thus, if distances between the drive electrodes and the detection electrodes are short, then a coupling capacitance between the drive electrodes and the detection electrodes becomes so large that it is difficult to perform the detection with high accuracy.
Further, if distances between the drive electrodes and the detection electrodes are set to be longer in order to reduce a coupling capacitance produced between the drive electrodes and the detection electrodes, then it is necessary to reduce an area of the drive electrodes so as to compensate the long distances between the drive electrodes and the detection electrodes. Accordingly, a driving voltage becomes high.